1. Field of the Invention
The present invention generally relates to acceleration sensors (accelerometer) for sensing acceleration. More specifically, the present invention is directed to a semiconductor acceleration sensor in which an acceleration detecting circuit is arranged by employing a semiconductor strain gauge, and also to a method for testing the acceleration sensor.
2. Description of the Prior Art
A compact semiconductor acceleration sensor arranged by assembling a strain gauge into a semiconductor substrate is known in the field. Normally, testing of semiconductor acceleration sensors is carried out by utilizing a large-scaled mechanical vibration testing machine. To manufacture a semiconductor acceleration sensor having a uniform sensing characteristic, such an adjusting method has been employed to adjust sensor sensitivities obtained by using the vibration test (namely, acceleration test) on a mechanical vibration testing machine with employment of a correction circuit. However, since a plurality of mechanical vibration testing facilities are operated in a parallel mode so as to perform these sensor testing processes in a mass production, lengthy testing time is necessarily required and furthermore the manufacturing cost of the semiconductor acceleration sensor is increased.
To solve the above-explained problems, one conventional testing method has been proposed in U.S. Pat. No. 5,103,667 issued in 1992 to Allen et al. FIG. 1 schematically shows an example of the semiconductor acceleration sensor as disclosed in this U.S. patent. The acceleration sensor is constructed of silicon mass (weight) 110, a cap 140, and a silicon base 150. The silicon mass 110 is supported via beams (flexible portions) 112 and 114 by a silicon frame 120. Two piezoresistors 130 and 132 are formed on the upper surfaces of the beams 112 and 114. The cap 140 is positioned opposite to the frame 120 in order to define an air gap 142, and a displacement electrode 160 is provided at the inner surface of the cap 140. The mass 110 is located opposite to the silicon base 150 in order to constitute another air gap 152. A pad 161 is provided on the upper surface of the frame 120. This pad 161 is electrically connected to electrode 160 by way of a metal conductor 180 formed on the surface of the cap 140.
The silicon base 150, the silicon frame 120, and the silicon cap 140 are jointed with each other, or adhered to each other. Even if these components are jointed with each other by solder, or adhered to each other by using an adhesive agent, any of these adhesive layers and joint layers would deteriorate, resulting in poor reliability. There is another problem that the thickness of either the joint layer, or the adhesive layer must be controlled.
As the method for jointing the members without any joint (adhesive) agent, the electrostatic joint method (anode joint method) is known in the art. This electrostatic joint method is used to joint silicon with glass in accordance with the following manner. That is, silicon and glass are in close contact with each other. Under heating at temperatures from 300.degree. to 500.degree. C., approximately 500 volts are applied to these silicon and glass connections, so that alkali ions contained in the glass are transferred and a space-charge layer is produced near boundaries between the glass and the silicon. As a result, a large electrostatic force is generated between the surface of the silicon and the glass, whereby chemical bond can occur at the boundaries. It is considered that the chemical bond would be caused by either deformation of the glass by receiving the electrostatic force, or by such a reason that O.sup.-- (ion) contained in the glass is transferred by receiving the electric field, and then bonded with Si (silicon), whereby SiO.sub.2 (silicon oxide) is formed at the boundaries.
In case that the electrostatic joint is employed to joint silicon to silicon as in the above-identified U.S. patent, an SiO.sub.2 film is formed on both surfaces of the silicon to be jointed by way of wet oxidation. At the same time, a large amount of SiOH radical is formed in the SiO.sub.2 film. This is because H.sup.+ caused by the following reaction formula is used as a carrier: EQU SiOH.fwdarw.SiO.sup.-- +H.sup.+
Since the electrostatic joint of silicon to silicon is carried out at such a high temperature, for example, 900.degree. C., aluminum which is normally employed as a wiring pattern in a semiconductor device could not be used. Furthermore, there is another adverse influence caused by heating such a device in which the circuit has been fabricated under higher temperatures. Practically speaking, it is difficult to apply the above-described electrostatic joint process to the semiconductor acceleration sensor.
Therefore, it is conceivable that glass is used instead of such silicon to form the cap 140 disclosed in the above-explained U.S. patent, and this glass cap 140 is electrostatically jointed with the silicon frame 120 under low temperatures from 300.degree. C. to 500.degree. C.
In FIG. 2, there is shown an example where both of a cap 200 and a substrate 300 are made of glass, and such a triple layer structure with a silicon detecting member 100 is electrostatically jointed under a low temperature. FIG. 2(a) is a sectional view of an overall acceleration sensor, and FIG. 2(b) is a partially enlarged diagram of this acceleration sensor.
A conductive film 202 is formed in a concave 201 of the cap 200, and a wiring pattern passes from this conductive film 202 via either a joint surface, or an adhesive surface 400 to the silicon detecting member 100 for connection purpose. Here, a Wheatstone bridge is formed on the upper surface of a beam 101 by a semiconductor strain gauge 104, and both the silicon detecting member 100 and the weight 102 are connected to the ground line of this bridge circuit. A supporting member 103 of the silicon detecting member 100 is electrostatically jointed to a substrate 300, thereby forming a concave 301.
To realize such a structure of one conventional semiconductor acceleration sensor, the wiring pattern must be formed from the concave 201 of the cap 200 through either the joint surface, or the adhesive surface 400 to the silicon detecting member 100. As a result, it is technically difficult to maintain flatness of either joint, or the adhesive surface 400, which will deteriorate the reliability of the mechanical strength. In particular, when such an acceleration sensor would be utilized as crash detecting sensors, e.g., automobile air-bag devices, the above-described deteriorated reliability would cause serious sensor problems because the sensor structure would require high reliability. On the other hand, although it may be practically possible to make either the joint area, or the adhesive area larger in order to avoid the above-described problems, there are other different problems that the chip size of the acceleration sensor would become larger, and the manufacturing cost thereof would be increased.